144 RECONNAISSANCE FROM CARROLL, MONTANA, 
We know of no described fossil shell very closely resembling this one. 7. Wyomingensis 
Meek is perhaps the most closely related, but differs conspicuously in the position of the beaks, 
which, in that one, are situated only about one-fourth of the length from the anterior end, while in 
this they are nearly central. ° 
Locality and formation.—In Cretaceous strata near the mouth of the Judith River, Montana, 
in beds apparently overlying the Fort Pierre sbales. 
Genus MACTRA, Linn. 
MACTRA MAIA,N. sp. 
Plate 2, fig. 5. 
Shell small, subtriangular in outline, with moderately convex valves. Anterior and posterior 
cardinal slopes nearly equal, the anterior side a little the longest and less abrupt; concave between 
the beak and the anterior end, while the posterior margin is convex. Anterior extremity narrow, 
rather strongly rounding upward from the basal margin; posterior extremity subangular; basal 
line very convex, slightly emarginate just within the posterior angle; beak short, broad, and 
obtusely pointed, the apex minute, curving, and closely appressed. Body of the shell somewhat 
regularly convex from beak to base, marked by a strong, subangular, posterior umbonal ridge, 
behind which the shell slopes abruptly to the margin, and just within which there is a very faintly 
depressed sulcus extending from below the umbo to the basal line. Anterior umbonal ridge 
rounded and abrupt. 
The specimen from which the description is taken is a partial cast, so that the surface is not per- 
fectly seen; it appears, however, to have been nearly smooth, or with only fine lines of growth. The 
hinge characters are not clearly made out; the posterior lateral tooth, however, is seen to be long 
and slender, reaching nearly one-half of the distance between the beak and postero-basal angle: 
The pallial sinus is somewhat rounded, slightly directed upward, and extends nearly to, or more than 
one-third of the length of the shell from the posterior end. 
This species is very similar in general expression to J/. incompta White, MS., but differs in 
being longest anterior to the beaks, while the reverse is the case with that species. 
Formation and locality —tIn beds of the Cretaceous formation believed to overlie the Fort Pierre 
shales near the mouth of the Judith River. 
Genus SANGUINOLARIA, Lam. 
SANGUINOLARIA OBLATA, D. Sp. 
Plate 2, figs, 3 and 4. 
Shell swall, transversely broad-elliptical or suboval, widest anterior to the middle of the length, 
where the width is equal to about two-thirds of the length; extremities broadly rounded, the pos- 
terior one most sharply curved; basal margin strongly rounded, most abruptly so anterior to the 
middle of its length; cardinal margin much less strongly rounded than the basal border, slightly 
contracted posterior to the beaks, which are small, compressed, and but slightly projecting beyond 
the cardinal border. Surface of the left valve very depressed-convex, most strongly curved across 
the shell from beak t base, and, judging from the form, has been more convex than the right 
valve; posterior end marked by a very faint sulcus passing from behind the beaks to the postero- 
cardinal margin. 
Surface of the shell marked by five concentric undulations and finer lines of growth. 
Formation and locality.—In sandy limestone of Cretaceous age near the mouth of the Judith 
River, overlying the Fort Pierre shales. 
Genus THRACIA, Leach. 
THRACIA (CORIMYA) GRINNELLI, 0. sp. 
Plate 2, figs. 6 and 7. 
Shell of medium size, transversely broad suboval, nearly equilateral, slightly inequivalve, and 
apparently a little gaping posteriorly. Basal margin of the shell forming a regular elliptical curve 
